1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a solvent-free coating that is solid at ambient temperature (hereinafter referred to as “solid paint”).
2. Description of Related Art
Typically, the term “solid paint” includes coatings that are polymer-based and, by way of example, have a resin, a pigment and a cross-linking agent as the main constituents of the coatings.
It will be understood that the term “solid paint” includes coatings that do not have sharply defined melting points (and the precise state of the coatings depends on the exact ambient temperature) but which are solid at most ambient temperatures.
It will also be understood that the term “solvent-free” includes coatings that contain small amounts of residual solvents (for example residue from the manufacturing process).
Examples of solid paints are powder coatings and paints exemplified in Australian patent 651007.
It is difficult to manufacture small quantities of solid paint efficiently and quickly. The reason for this is that solid paint cannot readily be manufactured by the conventional route that is often used for small quantities of solvent-based and water-based paints. These liquid coatings often make use of bases or tinters to allow small quantities to be made quickly, efficiently, with low loss, and at low cost. Until now, this manufacturing technique has not been available for solid paint. As a consequence, conventionally, solid paint is manufactured in small batches on apparatus that is designed for large batches, with the result that there are long equipment clean-up times at colour changeovers and proportionally large losses of paint at such changeovers.
One attempt to overcome the difficulty of manufacturing small quantities of solid paint efficiently and quickly involves grinding powdered paint to a very small particle size and combining the powdered paint with coloured powders and agglomerating the mixed powders to produce a paint with the required visual colour. This is described in Australian patent 643191.
This proposal suffers from the disadvantage of considerable expense of grinding powdered paint to very small particle sizes and thereafter agglomerating mixtures of ground powdered paint and coloured powders to required product size.
In addition, processing solid paints that are thermosetting paints in accordance with this proposal is especially difficult since thermosetting paints are liable to change chemically if the processing temperature exceeds the cross-linking temperature of the paints. This is a significant issue since solid paints are usually thermosetting paints.
Another attempt to make small quantities of solid paint is disclosed in U.S. Pat. No. 6,130,273. The method disclosed in the US patent includes the steps of extruding a high glass transition temperature (Tg) resin, pigments and any other solid compounds (such as cross-linkers and flow modifiers), melt blending the extrudate and a low Tg resin (liquid at ambient temperature) to form a mixture, and allowing the mixture to cool.
While this method may produce a coating with useful properties, it is not a manufacturing method which can easily and efficiently manufacture small batches, but rather is appropriate for large batches. Us of an extruder to incorporate the liquid resin into the molten coating as a last stage presents the difficulties of cleaning and wastage and lengthy delays when required to manufacture small batches of different colours often. In addition the method requires other equipment such as a powder mixer. It is difficult in a manufacturing environment to schedule all the equipment to be available to respond to manufacturing requirements quickly.
Another attempt to make small quantities of solid paint involves using a mixer know as a Z-Mixer. This mixer consists of a mixing vessel fitted with one or more stirrer blades, each of which are in a Z shape and rotate about a horizontal axis. This type of mixer finds common use for dispersing solid particles in a liquid to form a high viscosity paste. (eg. for kneading dough in a biscuit manufacture). However it is not appropriate for producing solid paint which requires bulk melting of the solid rather than dispersing small solid particles. It suffers from slowness in infusing heat into the mix (because the stirrer blades cannot move quickly) and consequently long processing time. It is also extremely difficult to clean. Both these key problems make it inappropriate for efficient manufacture of small batches of sold paint.
Another attempt to make small quantities of solid paint involves using a Banbury mixer. Banbury mixers are commonly used in the rubber industry to incorporate material into viscous media. The mixers have two interrupted spiral rotors moving in opposite directions to produce a high shearing zone. The mixers are not appropriate for manufacturing small quantities of solid paint as cleaning times are lengthy and product losses are high.
An object of the present invention is to provide a method of manufacturing solid paints that has advantages over and is not subject to the disadvantages of the above-described methods.